The present invention relates to substrates used in the fabrication of electronic devices, and more particularly to substrates which are suitable for the application of thin metal films thereon.
Increasing numbers of microelectronics and wireless devices require extremely small, extremely reliable electronic components and interconnections, both passive and active, which may be embedded and/or on the surfaces of such devices. Such surface interconnections are typically formed by using thin film metallization techniques to apply conductive traces onto the surface of a substrate. However, these techniques cannot reliably be carried out when the surface roughness of the substrate is greater than about 15 microinches (3750 Angstroms). The term xe2x80x9csurface roughnessxe2x80x9d, as used herein, refers to the average difference in the heights of the topographical features within a given area on the surface of a substrate.
Several methods for creating a smoother substrate surface are known. For example, one can polish a substrate to achieve the desired surface finish. However, for some ceramic substrates, the polishing forces can drag constituents of the ceramic out of the matrix, resulting in pitting of the surface.
It is also known to apply a glass layer over the surface of the substrate so as to provide a smooth surface. Such a glass layer establishes a relatively uniform surface topography and is typically applied as a relatively thick paste or slurry, which flows into voids and around peaks on the surface of the substrate, and is then fired to form a glaze. U. S. Pat. Nos. 3,968,193 to Langston, Jr. et al., 5,097,246 to Cook et al., and 5,686,790 to Curtin et al. all disclose such methods.
Alternatively, a thin glass layer can be deposited onto the surface of a substrate by chemical vapor deposition methods and then caused to flow viscoelastically over the surface of the substrate. U. S. Pat. No. 5,104,482 to Monkowski et al. discloses a method wherein a glass layer is deposited onto a substrate and simultaneously flowed viscoelastically over the surface of the substrate so as to planarize the surface of the substrate.
The prior art suggests that it is desirable to fill in or otherwise planarize the inherent surface topography of a substrate to eliminate peaks, valleys and voids in the surface. Glass deposits, as well as glass pastes and slurries, are applied to fill in the surface topography to form a nonconformal coating. However, the prior art does not appear to disclose or suggest the application of a substantially conformal layer or film of glass to the surface of a substrate to prepare the substrate for thin film metallization, without accompanying viscous flow of the glass, and thus without significantly altering the topography of the substrate.
It would therefore be advantageous to provide devices which are suitable for thin film metallization but which do not require a nonconformal coating or the application of a relatively thick glass paste or slurry. It would also be advantageous to provide methods for preparing a substrate for thin film metallization which do not require the use of thick glass pastes or slurries and which do not require glazing of the substrate.
According to one aspect of the invention, there is provided a method of surface preparation of a substrate for thin film metallization. The method comprises the steps of providing a substrate having a given surface topography, treating the substrate to remove substantially all surface contaminants and moisture from the substrate, and applying one or more substantially conformal glass films or layers to the surface of the substrate.
The term xe2x80x9cfilmxe2x80x9d, as used herein, refers to a substantially conformal layer of glass which is deposited onto a substrate using known chemical or sputter deposition techniques.
In one embodiment, the substrate may comprise, for example, a cofired ceramic material which may be either a low-temperature or a high-temperature cofired material. Other substrate materials can also be used.
Each glass film layer has a thickness of up to approximately 12,000 to 15,000 Angstroms, and preferably between about 4000 to 6000 Angstroms. Multiple glass film layers of between about 4000 and 6000 Angstroms may be applied. Preferably each glass film layer has a substantially uniform thickness.
The substrate is preferably treated by washing it several times in acid, alcohol and deionized water, and drying it under sufficient heat to remove substantially all moisture in the substrate.
In a preferred embodiment, the conformal glass film or films are applied using chemical vapor deposition techniques.
According to another aspect of the invention, a substrate which is suitable for receiving thin metal films in a predetermined pattern comprises a substrate having a given surface topography, and one or more substantially conformal glass films on the surface of the substrate.
The substrate can be, for example, a cofired ceramic which may be either a high-temperature or a low-temperature cofired ceramic material. Other substrate materials can also be used.
Each glass film layer has a thickness of up to approximately 12,000 to 15,000 Angstroms, and preferably between about 4000 to 6000 Angstroms. Multiple glass film layers of between about 4000 and 6000 Angstroms may be applied. Preferably each glass film layer has a substantially uniform thickness.
These and other objects and advantages of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, the scope of which will be indicated in the claims.